Increased levels of storage capacity in floppy and hard disk drives are a direct result of the higher track densities possible with voice-coil and other types of servo positioners, as well as the ability to read and write narrower tracks by using, for example, magnetoresistive (MR) head technology. Previously, low track density disk drives were able to achieve satisfactory head positioning with leadscrew and stepper motor mechanisms. However, when track densities become so great that the mechanical error of a leadscrew stepper motor combination is significant compared to track-to-track spacing, an embedded servo is needed so that the position of the head can be determined from the signals it reads.
Conventional servo-patterns (also referred to as servo-data) typically include short bursts of a constant frequency signal, very precisely located offset from a data track's center line, on either side. The bursts are written in a sector header area, and can be used to find the center line of a track. Staying on center is desired during both reading and writing. Since there can be sixty, or even more, sectors per track, that same number of servo-data areas must be dispersed around a data track. These servo-data areas allow a head to follow a track center line around a disk, even when the track is out of round, as can occur with spindle wobble, disk slip and/or thermal expansion. As technology advances to provide smaller disk drives, and increased track densities, the placement of servo-data must also be proportionately more accurate.
One example of servo-data is shown in FIG. 1, which includes a sector header 2 followed by a pattern to provide radial position information. The sector header includes a Servo ID (SID) Field 4 and a Grey Code Field 6, which require precise alignment track to track. Misalignment in these patterns results in destructive interference of the magnetic pattern and reduces the amplitude of the signal which leads to errors. Specifications on the alignment in modern disk drives is approximately 25 nanosec (3 sigma) track to track for a disk rotation period of roughly 11 milliseconds or 2.3 ppm. This narrow time window therefore requires precise measurement of the disk angular position over many revolutions of the disk.
As disk drives become smaller and track densities increase, there is a desire to reduce the size of the servo-data areas, such that they take up less space on the disk. In order to reduce the size, however, the servo-data are written at higher and higher frequencies. These higher frequencies require tighter timing tolerances from track to track.
In one example, timing is provided by writing trigger patterns at various locations of the disk. It is understood that in writing a trigger pattern a specified time after a trigger, the presence of electronic delays in the trigger and write circuitry is taken into consideration. This is described in IBM Technical Disclosure Bulletin, Vol. 33, No. 5 (October 1990), where the delay between A and B clock areas is measured and stored. This delay value is used to advance the write timing of all subsequent servo-tracks and clock areas.
Although, the IBM Technical Disclosure Bulletin, Vol. 33, No. 5 (October 1990) discussed the presence of electronic delays, it did not discuss how to achieve optimum track to track trigger pattern alignment in the presence of systematic errors (e.g., constant for every sector), which vary as a function of radial position and/or circumferentially in the propagation process.
Therefore, a need still exists for a capability to reduce systematic errors in the writing of timing patterns. In particular, a need exists for an improved capability to:                1) Eliminate a varying systematic error when the recording head has spatially separate read and write elements, such as is the case for magnetoresistive heads. This results in a read to write time delay which is radially dependent.        2) Remove the varying systematic error due to a read element and write element, which are non-parallel, resulting in an error in the measurement and subsequent writing of trigger patterns.        3) Eliminate servo pattern rotation due to residual or unmeasured systematic errors by using a once per revolution clock index derived from the motor drive current waveform or any other sensor.        4) Reduce systematic errors which vary circumferentially.        5) Minimize the error due to a recording head mounted non-parallel to the actuator motion.        